1. Field of the Invention
The present invention relates to a fixing member and a fixing apparatus used for an image forming apparatus such as an electrophotographic copier, printer, or facsimile terminal equipment and to an electrophotographic apparatus making use of the fixing member and the fixing apparatus.
2. Related Background Art
A copier, which is an electrophotographic apparatus, has a fixing apparatus mounted therein to fix a toner image which is transferred onto a sheet such as recording paper or a transfer material, which is a recording medium. The fixing apparatus has, for example, a fixing roller also called as a heating roller that thermally melts a toner on a sheet, and a pressurizing roller that is brought into pressure contact with the fixing roller to sandwich or pinch the sheet therebetween.
The fixing roller is formed to be hollow, and on its central shaft, a heating element is held by a holding means.
The heating element is composed of, for example, a tubular heater such as a halogen lamp and generates heat when a predetermined voltage is applied to it.
Since the halogen lamp is located on the central shaft of the fixing roller, heat from the halogen lamp is radiated uniformly to the inner wall of the fixing roller, and the temperature distribution in the outer wall of the fixing roller is uniform in the circumferential direction.
The outer wall of the fixing roller is heated until its temperature reaches a value suitable for fixation of images (for example, between 150 and 200.degree. C.). Under this condition, the fixing and pressurizing rollers are rotated in the opposite direction while being mutually pressure-contacted in order to sandwich therebetween a sheet with toner deposited thereon. The toner on the sheet is melted at the pressure-contacted portion (hereafter also referred to as a "nip") between the fixing and pressurizing rollers due to heat from the fixing roller and is fixed onto the sheet due to pressure effected by both rollers.
However, the fixing apparatus including a heating element composed of, for example, a halogen lamp uses heat radiated from the halogen lamp to heat the fixing roller, so a relatively long time is required from power-on until the temperature of the fixing roller reaches a predetermined value suitable for fixation (hereafter referred to as "warm-up time"). During this time, the user cannot use the copier and must wait over a long time.
On the other hand, if a large amount of electric power is applied to the fixing roller to improve user operability in order to reduce the warm-up time, the power consumption of the fixing apparatus increases to contradict efforts to save energy.
Thus, to improve the value of products such as copiers, more efforts have been made to achieve both energy saving for the fixing apparatus (reduction of power consumption) and the improvement of the user operability (quick print).
As an apparatus that meets such a requirement, a fixing apparatus based on the induction heating method and using high-frequency induction as a heating source has been proposed as shown in Japanese Patent Application Laid-Open No. 59-33787. This induction heating fixing apparatus comprises a coil located concentrically inside a hollow fixing roller consisting of a metal conductor so that high-frequency magnetic fields are generated when a high-frequency current is caused to flow through the coil and the magnetic fields generate an induction eddy current in the fixing roller, which is then Joule-heated due to its skin resistance.
This fixing apparatus based on the induction heating method significantly improves the electricity-heat conversion efficiency to enable the warm-up time to be reduced.
In this fixing apparatus, however, the temperature around the coil substantially increases due to heat radiation to the inner surface of the fixing roller, thereby increasing the electric resistance of the coil and thus required power.
In addition, the coating of the coil consisting of a resin is melted by heat, thereby degrading the insulation of the coil.
These problems also apply to a type that uses as a heating member a thin flexible metal sleeve instead of the fixing roller.
Thus, to restrain an increase in the temperature of the coil, the provision of a cooling mechanism such as a blowing means inside the fixing roller has been proposed as disclosed in, for example, Japanese Patent Application Laid-Open No. 54-39645.
If, however, a cooling mechanism is installed in the conventional fixing apparatus based on the induction heating method, an installation space is required for this mechanism, resulting in the increased size of the apparatus and thus increased costs.
In addition, not only the induction coil and the core but also the inner surface of the fixing roller are cooled by air blow, thereby degrading the fixing capability.
In addition, to restrain an increase in the temperature of the coil, the provision of a low thermal conductive layer formed of, for example, polyimide or polyamide imide in the inner surface of the fixing roller has been proposed (Japanese Patent Application Laid-Open No. 7-114276).